The present invention relates to a PTC (positive temperature coefficient) composition which comprises a thick-film type PTC element.
Conventional thick-film type PTC elements are usually formed from polymers and have conductive particles dispersed in the polymer. The types of polymers used include non-crystalline vinyl polymers, side-chain crystalline vinyl polymers, and crystalline polymers with high melting points.
These conventional PTC elements resemble the one shown in FIG. 1. In FIG. 1, the PTC element body 3 is formed on substrate 2 with a pair of electrodes 1 affixed thereto. A lead wire terminal 4 is connected to each electrode.
A PTC element increases its resistance as the temperature rises. Referring to FIG. 4, as temperature T rises to the glass-transition temperature (Tg) of the polymer material of which the PTC element is made, the resistance of the PTC element gradually increases. The increase in resistance occurs because as the temperature rises, the polymer in the PTC element experiences micro-Brownian motion. The resulting expansion of the polymer tends to separate the conductive particles. The separation of the conductive particles produces a proportionate increase in resistivity. When the temperature reaches the glass-transition temperature, the polymer begins to undergo inter-molecular motion which considerably increases the volume of the polymer. This increases the distance between the conductive particles present in the polymer and results in a sharp increase in the resistance.
In the prior art, a non-crystalline vinyl polymer has been used in the PTC composition. In this case, the PTC composition is formed by first grafting the non-crystalline polymer to the surfaces of carbon black particles by solution polymerization. Next, cross-linking occurs by adding an epoxy resin as a cross-linking agent. The composition is then heated and made into a thick film. The resulting composition is a non-crystalline vinyl polymer PTC composition.
The prior art also discloses the use of side-chain crystalline vinyl polymers to form the PTC composition. The use of this polymer is disclosed in A New Composite Register With PTC Anomaly (J. Polymer Sci. 19. 1871 (1981) by K. Ohkita, et al). It requires that carbon black particles be dispersed in a side-chain crystalline vinyl polymer in solution to form the PTC composition.
A still additional polymer that has been used in the prior art to form PTC compositions is a crystalline polymer with a high melting point. The specific type of crystalline polymer usually used is polyethylene. The PTC composition is formed by grafting the crystalline polymer to the surfaces of carbon black particles by thermal mixing.
There are problems in the prior art when side-chain crystalline vinyl polymer is used to form the PTC composition. The carbon black particles are not thoroughly dispersed in the side-chain crystalline vinyl polymer because the polymer is not grafted to the surfaces of the carbon black particles. This results in widely varying resistance values inside the PTC element body. Varying resistance values result in varying temperature rises, including localized hot spots.
To avoid the problems above, non-crystalline vinyl polymer is normally used in the PTC composition to form a thick-film type PTC element.
The ideal PTC element exhibits a constant device temperature response, steep cut-off current characteristics, and large current limiting function at the polymer's glass transition temperature (Tg). These results are obtained where there is a large rate of increase of resistance and a steep rise in resistance at the initiation of PTC behavior.
Prior thick-film PTC compositions of non-crystalline vinyl polymer have not exhibited the ideal characteristics outlined above. Instead, their PTC behavior is exhibited at the glass transition temperature (Tg) of the cured non-crystalline vinyl polymer. As a result, the rate of increase of resistance is small and the rise in resistance at the initiation of PTC behavior is gradual. Additionally, the PTC composition has a large value of resistance which makes miniaturization difficult.